Optical pyrometer



- y 29, 1930- c. E. FOSTER 1,771,533

OPTICAL PYROMETER Filed Nov. 6,. 1926 II E 4 a Patented July 29, 1936 oHAitLEs nnwmrosrnn, or LEroHWoRri-r, ENGLAND OPTICAL PYROMETER Application filed November 6,1926, Serial No. 146,738, and in Great Britain November 19, 1925'. l

y This invention relates to optical pyromet- .ersof the disappearing filament type.

, Inthe simplest form-of this type of in strument current from a battery is passed through a variablerheostat, the filament to be heated, and a milliammeter. Bythe position of thecontact on the rheostat the current to the filament is-varied to secure the desired matching of the filament against the hot bodythe temperature of which is to be measured. Such temperature is measured directly on the scale of themilliammeter.

.. With this simple circuit all that portion of the scale of the milliammeter is useless below the lowest reading represented by the distance the milliammeter needle travels under the influence of the current required to be passed through the filament to heat the same up to the lowest temperaturereading on the milliammeter scale. One method of overcoming this disadvantage'wasto employ .the device of a mechanical set back of the zero so as to make the temperature range occupy the whole scale. By the use of such a device there was no means of checking the true zero and thereforethere existed an element of uncertainty and possible error by. reason of the fact that the set backim-' posedalarge and continuous torsion on the instrument springs and such could therefore not be relied upon to be free from creep and change in'zero. Y This difiiculty has been surmounted by making the lamp filament one of the arms of a Wheatstone bridge and using'the milliammeter asthe bridge galvanometer in themanner described in U. S. patent to Schofield 1,560,864, of November 10, 1925.

The object of the present invention is'toprovide a further and simple method' of overcoming the scale diificulty. 7

According to the present invention I overcome the difiiculty by providing means whereby the turning movement of the coil of an. indicating instrument having an indicating finger carried by a coil moving in the poles;

of a permanent magnet, such as by making the magnetic field of a moving coil of a mil liammeter definitely non-uniform or by using a voltmeter across the lamp terminals, so

- OFFICE? that the first increments of current only produce a predetermined and reduced deflection co l; Thls may be eflfectedin practice when using a milliammeter as the indicating i11- strument by suitably cutting away one pole control springs being then arranged so that they have a non-uniform law of resistance to the rotation of the moving coil.

With'the cut away pole pieces the first increments of current would. only produce proportionately smaller deflections of the coil so that the position occupied bythe first temperature division will be muchnearer to zero and there would thus be a larger'proportion of the scale length usefully employed.

However eiiected it is desirable according to the present invention that an indicating instrument be used in which thefmovement of the pointer for a given increment of current in the neighbourhood of zero is not more than two thirds of the movement for a similar increment near the centre of the scale.

In practice the construction can be such as will result in the sensitivity of the instrument at the centre of the scale being at least greater than the sensitivity at the beginning of the scale, that is to say the angular deflection for a given current in the neighbourhood of zero would be not more than two-thirds of the angular deflection for a similar increment of current at the centre of the scale.

In an instrument according to this invenmoved pointer. By fitting this additional holewith an eye piece the scale of the instrument, can be magnifiedand brought to .an' apparent greater distance so that the user ing one way of carrying the invention into "a voltmeter is used as the meter having a scale 5.

has a'unilorm scale thecurrent $01 700" "0.44 "ampere while that for 1200 C; is :62

effect-viz, Eby suitably shaping e01 cutting away two diametrically opposite pole horns. of the millia'mmet-er; x V

Figure is a diagram of the circuit when an an-nneter is used as theindicating instinmerit and Figure 6 a similar'diagram when indicating instru- 'ment.

" *In'the example illustrated in Figures 1 to 3 the objective lens of "the telescope isintlicated at 1 and "the eye-piece .atQ, the lamp filament being arranged intermediately and indicated at 3, the usual rheostat 12, controlling the current to such filament being operatedby the knob 4.

.In this example the milliammeter is inounted directly upon the tube of the telescope, its

scale, indicated at 5, being situated to one side of such tube. "So that "the scale 5 may be readwithout taking the eye oil? the filament 3, an additional eye-piece 6 is provided such eye-piece being incorporated with an eye-shield 7 icommonto both eye-pieces 2 and 6. The'v'iew obtained when looking through both eye-pieces is illustrated in Figure 3.

' shown in Figure ithe pole horns 8 and p '9 are cut away as atlO so that portions of them will be at gradually increasing distances "from the axis about which turns the coil '11 carrying'the indicating finger 17, this 'producingthe effect that the first increments of current only produce a predetermined and reduced deflection of the coil'lll In the diagram of the circuit shown in Figure 15, 1-3 is a battery {12a variable resistance,8 a filament lamp and 14 a milliam- Figure '6 is a similar diagram with 2. voltmeter calibrated as the indicating instrument in place of the milliammeter, 13 is a battery,

12 a variable'resistance, '3 ca filament lamp and 1 5 a voltmeter having a scale *5.

"The method illustrated by Figure 6 has the advantage that owing to the increase in the resistance of the filament itself the law of change of voltage is steeper than that of changes of current. i 1 This may be explained numerically as folows Assuming that the instrument employed ampere. With this measurement of current the useless fraction of the scale is 0.44; divided by 0.62 which equals 71%. If voltage is measured instead of current, the Voltage corresponding to 700 C. is 0.47 5 and that to 1200 C. is 0.860. The useless portion of the scale which is then 0.47 5 divided by 0.860 only represents-%. -If an instrument is used having a non-uniform scale there will still be the same relative advanta e. 7

What I claim and desire to secure by LettersPatent is 1. An optical pyrometer having in comination indicating instmmentH-having a calibrated scale over which a finger carried by the coil moves, thecoil moving between the polesoi a permanentmagnet, means for varying the magnetic field to produce a non-uniiormtmovement of the -indicating finger during its initial movement, a telescope, two eye pieces, an "electric filament lamp, a battery, means for regulating cur-rent throu h the lamp from the battery, the indicating instrument being mounted in the telescope with its scale under one'eye piece, and the electric filament lamp between the other eye piece and the objective of the telescope.

. 2. An optical pyrometer having in combination a voltmeter having a calibrated-scale over which a finger carried by the coil moves, 9 the coil moving between the poles of a permanentmagnet, a telescope, twoeye pieces, an electric filament lamp, a battery, a rheostat in the battery and lamp circuit, the voltmeter being mounted in the telescope with its scale under one eye piece, and the electric=filament lam-p between theothereye piece and the'obijrect'ive of the telescope.

An optical pyrometer having in combination an indica tingfinstrument having a calibrated scale over whicha finger carried by the coil moves, the coil moving between the polesoi a'permanent magnet, the air gap between the moving coil and "the magnet being greater at one end of 'the movement of the coil than at the other, a telescope, two eye pieces, an electric filament lamp, a battery, means for regulating current through the lamp from the battery, the indicating instrument being 'lIlOllHliGdZl-Il the 'telescope'wi-th its scale under one eye piece, and the electric filament lamp between the ofiher'eye 'pieceand the objective of the telescope.

In witness whereoil allix my 'ature.

CHARLES EDW *[N :FOSTER. 

